The understanding of cancer has evolved rapidly over the last decade, particularly with discoveries regarding the role of physical factors, such as extracellular matrix (ECM) stiffness and cellular forces, in carcinogenesis. This research has shown that altered ECM stiffness is not just a symptom of tumors, but is now known to trigger the actual onset of and progression of malignancy. Another key finding established by the Co-Investigator of this grant, is that cellular traction stresses increase with increasing metastatic potential, suggesting that cell traction forces could be a biomarker for the likelihood of metastasis. Additionally, it has been found that (2D) collective behavior of cell populations can be significantly different from that of isolated cancer cells, and that cell migratory behavior in 3D matrices is significantly different migration on 2D surfaces. Although this has motivated the adoption of 3D microenvironments in cancer mechanobiology research, current imaging methods to quantify ECM mechanical properties and local cellular forces only provide 2D imaging, or when they do support 3D imaging, they do not provide long-range volumetric measurements of collective mechanical behavior with cellular resolution. The central objective of this proposal is to demonstrate the feasibility of traction force optical coherence elastography (TF-OCT) for volumetric time-lapse imaging of cellular forces over distances that are long with respect to a single cell. We will utilize this to map local mechanical forces associated with individual and collective cancer cell migration in physiologically relevant assays for studying local invasion and early cancer metastasis in vitro. Aim 1 will develop and demonstrate the advantageous use of aberrated optical systems combined with computational image formation methods in OCT to enable cellular-resolution imaging over millimeter-scale volumes without having to scan the focus in depth. This aim will also develop 3D bead- tracking algorithms for OCT-based TFT. Aim 2 will apply this new imaging capability to image cell forces associated with single and collective cell migration in 3D microenvironments. These experiments will take advantage of a physiologically-relevant tumor spheroid assay developed by Co-Investigator Dr. Reinhart-King, and be patterned after these recent cell migration and ECM remodeling studies, but will now provide the volumetric time-lapse imaging of cellular forces. Imaging of the spatiotemporal forces associated with invasion dynamics of single/isolated highly metastatic and invasion-compromised breast adenocarcinoma cells, will be compared to collective invasion behavior at the boundary of tumor spheroid assays. This will include 3D force measurements of the ?leader-follower? cell behaviour recently observed by Dr. Reinhart-King?s group. This new volumetric time-lapse imaging capability could lead to a deeper understanding of potential physical (mechanical) hallmarks of cancer, that can be used in the future to design and test new ?mechano therapies? that target/modulate the mechanical properties of the ECM. The clinical compatibility of an OCT-based imaging will greatly enhance future efforts to translate cancer mechanobiology research to the clinic.